Where to Find
A good place to find electronics is the local thrift store. Many people just donate perfectly good equipment, accessories and cables if they don't need it. You can get expensive older stereos, speakers, computer equipment, monitors, televisions, cables, even old cellphones and appliances, all for cheap. Most good items will be sold quickly, so be sure to check back often. To get a leg up, and a discount, offer to volunteer in the electronics receiving area, testing equipment. Now you can get first pick of all the items donated, and access to some items that won't normally be put out on the floor. Come in once or twice a week and spend an hour testing anything that looks interesting to you. Chances are, you'll also be allowed to set the price of the items you test. Even without a little price gouging, you'll be able to get items that would sell for hundreds of dollars new for five or ten.
Never forget to check trash cans and dumpsters for easily repaired or unwanted electronics.
People often throw away slightly outdated electronics, so an interested party that invests their efforts dumpster diving can get quite a few goodies. For example: old computers will provide you with: power supplies, cases, free hard drives, cd drives, extra ram, cooling fans, heatsinks.
Audio equipment often gets thrown away in mostly working condition. Many people are too lazy to replace a power switch or an obvious damaged component (it's rarely an IC or anything difficult to see with the trained eye). People will throw away full stereos because one speaker is blown. Obviously we can find use for them, just look at Guerilla Audio.
Among other things that are useful to your local radical hackers: radio dishes, wall wart power supplies, light switches, lengths of wire, usb hubs, LED light bars... think: anything that can provide potentially useful parts is good.
Many sites will offer free samples of electronic components. http://www.google.com/search?q=order+samples Look!
Some people can put together audio amps for events, sensors for squats, radios... whatever.
An electrical or portable butane soldering iron, a electronic multimeter, and some basic knowledge will allow you to make money and influence people by fixing the often simple issues with their gadgets. Most often there is a breakage in a solder joint either near where a cable connects or there is mechanical stress look for cracks and test using the ohm function on your multimeter. The second most likely culprit is the power supply, this is either a power block or "wall wart" and external which you can easily replace after testing and finding it has failed, just be sure to get a replacement which outputs the right voltage and at least as many amps as the OEM supply. Some power supplies are internal but can usually also be replaced although you will have to guess at the rated amp/voltage output if it is blown.